Backscatter communication systems are known in the art. In an exemplary backscatter system, one transponder, such as an interrogator, sends out a command to a remote communications device. After the interrogator transmits the command, and is expecting a response, the interrogator switches to a CW mode (continuous wave mode). In the continuous wave mode, the interrogator does not transmit any information. Instead, the interrogator just transmits radiation at a certain frequency. In other words, the signal transmitted by the interrogator is not modulated.
After a remote communications device receives a command from the interrogator, the remote communications device processes the command. The remote communications device of the backscatter system modulates the continuous wave by switching between absorbing RF radiation and reflecting RF radiation. For example, the remote communications device alternately reflects or does not reflect the signal from the interrogator to send its reply. Two halves of a dipole antenna can be either shorted together or isolated from each other to modulate the continuous wave.
One example of a backscatter system is described in commonly assigned U.S. patent application Ser. No. 08/705,043, filed Aug. 29, 1996, and incorporated herein by reference. Another example of a backscatter system is described in U.S. Pat. No. 5,649,296 to MacLellan et al. which is also incorporated herein by reference.
One application for backscatter communications is in wireless electronic identification systems, such as those including radio frequency identification devices. Of course, other applications for backscatter communications exist as well.
Most presently available radio frequency identification devices utilize a magnetic coupling system. An identification device is usually provided with a unique identification code in order to distinguish between a number of different devices. Typically, the devices are entirely passive (have no power supply), which results in a small and portable package. However, such identification systems are only capable of operation over a relatively short range, limited by the size of a magnetic field used to supply power to the devices and to communicate with the devices.
Another wireless electronic identification system utilizes a large, board level, active transponder device affixed to an object to be monitored which receives a signal from an interrogator. The device receives the signal, then generates and transmits a responsive signal. The interrogation signal and the responsive signal are typically radio-frequency (RF) signals produced by an RF transmitter circuit. Because active devices have their own power sources. The active devices do not need to be in close proximity to an interrogator or reader to receive power via magnetic coupling. Therefore, active transponder devices tend to be more suitable for applications requiring tracking of objects that may not be in close proximity to the interrogator, such as a railway car.
In many applications, the path length intermediate the communicating devices of the system varies responsive to movement of the devices. Moving objects within the communications enviromnent or intermediate the communicating devices can cause communication disturbances. Further, some interference may bleed into the received signal of backscatter interrogators inasmuch as backscatter interrogators are configured to emit radiation while receiving data from a remote device. It is preferred to reduce or otherwise minimize noise and other disturbances to provide robust communications.